1. Field of the Invention
The present invention relates to a sheet stacking apparatus having a function of aligning sheets stacked on a stacking tray.
2. Description of the Related Art
Regarding a sheet stacking apparatus which is connected to an image forming apparatus and configured to stack a large amount of sheets, there has been a growing trend toward a requirement for a performance enabling sheets to be aligned with high accuracy before being discharged.
Japanese Patent Application Laid-Open No. 2006-206331 discusses an apparatus in which an alignment member is provided on a stacking tray, and the sheet end surfaces are aligned by attaching and separating the alignment member to and from the sheet end surfaces parallel to the sheet discharge direction to stack the sheets together.
However, in a case where sheets of a sheet width W1 are stacked on a stacking tray 701 and sheets of a sheet width W2 different from the sheet width W1 are stacked on the sheets of the sheet width W1, it is necessary to eliminate a gap between the already stacked sheets and the bottom surface of each alignment plate. For this purpose, as illustrated in FIG. 17, it is necessary for alignment plates A and B to abut the upper surface of the uppermost one of the sheets already stacked. When, in the state in which the alignment plates are in contact with the upper surface of the uppermost one of the stacked sheets, the alignment plate A moves in the directions indicated by an arrow in FIG. 17, the bottom surface of the alignment plate A is rubbed against the uppermost one of the stacked sheets. This leads to separation of the toner on the sheet, so that there is a fear of deterioration in image quality.
Further, when the bottom surface of the alignment plate with toner adhering thereto comes into contact with another sheet, the toner will be allowed to adhere to this sheet, so that there is a fear of deterioration in the image quality.
Further, even in a place where no toner image is formed, the sheet surface is rubbed against the alignment plate, and sheet quality may be deteriorated.